Transformation toughened ceramic alloys

ABSTRACT

This invention is primarily directed to the production of transformation toughened ZrO 2  -containing ceramic alloys wherein the toughening agent is selected from the group consisting of a rare earth metal vanadate and a magnesium and/or calcium tungstate/molybdate, and SnO 2  can be utilized as a toughening agent and/or a stabilizing agent for ZrO 2 .

BACKGROUND OF THE INVENTION

This invention relates to the transformation toughening of zirconia andother ceramic alloys by utilizing one or more stabilizer additives and atoughening agent. It focuses primarily on the use of novel tougheningagents in making a final product with high toughness.

Transformation toughening is associated with the volume change thataccompanies the tetragonal-to-monoclinic phase transformation. Thistransformation can be controlled by incorporating one or morestabilizing oxides into the ceramic matrix material, resulting inretention of the tetragonal phase from high temperature down to roomtemperature. Transformation toughened, tetragonal, partially stabilizedzirconia and ceramic matrix materials toughened by tetragonal, partiallystabilized zirconia have proven useful in areas where excellent thermalconductivity, hardness, toughness and strength are required, namely,wear/abrasion resistant ceramics, thermal shock resistant ceramics,cutting tools, draw dies, ceramic bearings, and oxygen ion conductors.

U.S. application Ser. No. 926,655, filed Nov. 4, 1986 under the titleTOUGHENED ZIRCONIA ALLOYS, provides an extensive discussion of ceramicalloys containing zirconia and/or hafnia as the primary component, withparticular emphasis being given to the mechanism involved in thetransformation toughening of zirconia and/or hafnia partially stabilizedwith yttria through the presence of niobate and tantalate compounds.U.S. Pat. No. 4,753,903 provides further discussion of transformationtoughened zirconia alloys wherein titania and yttria are included in thecomposition.

Thermal barrier coatings of zirconia stabilized or partially stabilizedwith yttria are currently utilized in gas turbine and other hightemperature heat engines. These coatings are degraded relatively rapidlythrough the loss of the yttria stabilizer due to the formation ofyttrium vanadate resulting from the presence of vanadium compoundimpurities in the fuels being used. The development of zirconia alloyscontaining a vanadium compound incorporated therein would yield bodiesexhibiting much increased resistance to that source of degradation.

Heretofore, the focus of stabilizer use has been to add a particularoxide dopant to zirconia to obtain specific crystal phase structures andphase distribution, and to arrest the transformation reaction at adefined stage of its operation. As a result, although early researchersworked with such oxides as MgO and CaO for stabilizing ZrO₂, more recentresearch has utilized exotic and costly additives in moderateconcentrations such as Y₂ O₃, CeO₂, and other rare earth oxides toproduce materials demonstrating many desirable properties. Needless tosay, the search for less costly stabilizing agents for ZrO₂ has beencontinuous.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a transformation toughenedZrO₂ and/or HfO₂ alloy through the incorporation of novel tougheningagents therewithin, the ZrO₂ and/or HfO₂ most preferably being partiallystabilized.

A second object of this invention is to provide a ZrO₂ and/or HfO₂ alloyexhibiting greatly increased resistance to attack by vanadium-containingcompounds.

A third object of this invention is to provide a transformationtoughened, partially stabilized ZrO₂ and/or HfO₂ alloy wherein theamount of rare earth oxide stabilizer and, if desired, the concentrationof ZrO₂ and/or HfO₂ can be substantially reduced through thesubstitution of a material as a stabilizer which is less costly than therare earth metal oxides, and which material, being compatible with ZrO₂,can replace a portion thereof, but without deleteriously affecting thedegree of toughening exhibited by the alloy.

Each of those objects can be specifically achieved through the selectionof particular materials, as will be described in detail hereinafter.Therefore, to illustrate:

The first object can be attained in:

A ceramic alloy demonstrating high fracture toughness consistingessentially, expressed in terms of mole percent on the oxide basis, of:

(A) 79-99.5% total of at least one member selected from the groupconsisting of ZrO₂, HfO₂, partially stabilized ZrO₂, partiallystabilized HfO₂, ZrO₂ -HfO₂ solid solution, and partially stabilizedZrO₂ -HfO₂ solid solution;

(B) 0.25-15% total of at least one stabilizer oxide in the indicatedproportion selected from the group consisting of 0-7% Sc₂ O₃, 0-7% Y₂O₃, 0-15% CeO₂, 0-15% TiO₂, and 0-7% RE₂ O₃, wherein RE₂ O₃ is a rareearth metal oxide selected from the group consisting of La₂ O₃, Ce₂ O₃,Pr₂ O₃, Nd₂ O₃, Sm₂ O₃, Eu₂ O₃, Gd₂ O₃, Tb₂ O₃, Dy₂ O₃, Ho₂ O₃, Er₂ O₃,Tm₂ O₃, Yb₂ O₃, and Lu₂ O₃ ; and

(C) 0.25-6% total of at least one toughening agent in the indicatedproportion selected from the group consisting of 0-6% MgWO₄, 0-6%MgMoO₄, 0-6% CaWO₄, 0-6% CaMoO₄, 0-4% WO₃, and 0-4% MoO₃.

U.S. Pat. No. 4,753,002, supra, discloses the efficacy of TiO₂ as astabilizing agent for ZrO₂. And, inasmuch as titanium and tin haveapproximately the same size ionic radii in the +4 valence state, theyboth can form oxides with crystals having a rutile-type structure, andthe solid solution between TiO₂ and SnO₂ is complete at hightemperatures. SnO₂ likewise can perform as a stabilizer in ZrO₂-containing alloys. A simple mixture of ZrO₂ and SnO₂, however, does notprovide the toughness desired in the product; a relatively small amountof Sc₂ O₃, Y₂ O₃, and/or a rare earth metal oxide is demanded to insuresignificant toughening. Nevertheless, the inclusion of SnO₂substantially reduces the required amount of ZrO₂ and stabilizer. Insummary, because of the above-described similarities in structure andbehavior, SnO₂ can replace TiO₂ in the alloys. In general, suchsubstitution will not exceed about one-half of the TiO₂ content.

The resulting alloy consists of fine-grained crystals of mixed phasestypically having diameters of less than about five microns consistingessentially either of tetragonal and monoclinic structures, or of aphase consisting essentially of tetragonal symmetry with a minor amountof a cubic phase and/or a minor amount of a magnesium and/or calciumtungstate and/or molybdate phase.

The second object can be attained in:

A ceramic alloy exhibiting resistance to attack by vanadium-containingcompounds consisting essentially, expressed in terms of mole percent onthe oxide basis, of:

(A) 65-99.5% total of at least one member selected from the groupconsisting of ZrO₂, HfO₂, partially stabilized ZrO₂, partiallystabilized HfO₂, ZrO₂ -HfO₂ solid solution, and partially stabilizedZrO₂ -HfO₂ solid solution; and

(B) 0.5-35% MVO₄, wherein M consists of at least one cation selectedfrom the group consisting of Mg⁺², Ca⁺², Sc⁺³, Y⁺³, Ti⁺⁴, and a rareearth metal cation selected from the group consisting of La⁺³, Ce⁺³,Ce⁺⁴, Pr⁺³, Nd⁺³, Sm⁺³, Eu⁺³, Gd⁺³, Tb⁺³, Dy⁺³, Ho⁺³, Er⁺³, Tm⁺³, Yb⁺³,and Lu⁺³.

In like manner to the above-described tungstate- and molybdate-toughenedalloys, Sn⁺⁴ can replace at least part of the Ti⁺⁴ content, normallysuch replacement will not exceed about one-half of the Ti⁺⁴ content.

Because vanadium is in the same group of the Periodic Table as niobiumand tantalum and has about the same ionic radius as those elements, itcan form compounds having crystal structures similar to those formed byniobium and tantalum. For example, yttrium vanadate has a zircon-typecrystal structure that can transform to a scheelite-type structure athigh pressures. YNbO₄, YTaO₄, and rare earth metal niobates andtantalates form tetragonal scheelite-type structures at hightemperatures that can transform to monoclinic fergusonite-typestructures during cooling to room temperature. CaWO₄ is the mineralscheelite and CaMoO₄ is the mineral powellite, both of which havetetragonal scheelite-type crystal structures at room temperature.

Therefore, because of these structural and chemical similarities, itwill be appreciated that, in like manner to the niobate/tantalatetoughening agents disclosed in Ser. No. 926,655, supra, yttrium vanadateand its rare earth metal analogs can form extensive solid solutions withZrO₂, and the crystal phases developed and the properties exhibitedthereby will be similar to those of the niobate/tantalate analogs.

At high levels of vanadate compounds, i.e., about 10-35%, anon-transformable crystal phase, adjudged to be of tetragonal structure,can form at temperatures of about 1100° C. and higher, in a mannersimilar to that exhibited by ZrO₄ -YNbO₄ solid solutions. Also presentwill be crystals exhibiting a cubic habit and crystals of a phase whosechemistry is dominated by the vanadate compound, but which crystals havenot been rigorously identified as having a cubic, tetragonal, ormonoclinic symmetry.

At lower concentrations, however, the vanadate compounds perform in asimilar manner to the above-described magnesium and/or calciumtungstates and/or molybdates to produce transformation toughened ZrO₂alloys. That is, the microstructure of the alloy will consist offine-grained crystals typically having diameters of less than about fivemicrons of mixed phases consisting essentially either of transformabletetragonal and monoclinic structures, or of a phase consistingessentially of transformable tetragonal symmetry along with a minoramount of a cubic phase. Accordingly, where a toughened body is desired,the content of vanadium compound will be maintained below 10% and,preferably, will not exceed 5%.

Both of the composition intervals, however, will demonstrate muchimproved resistance to corrosion by vanadium or vanadium compounds atelevated temperatures, because each is closer to chemical equilibriumwith vanadium compounds than are ZrO₂ -Y₂ O₃ alloys. The alloy with thegreater concentration of vanadate compound will display a higherresistance to such chemical degradation. In addition, thenon-transformable solid solution phase will not be as susceptible tospalling as is experienced in ZrO₂ -Y₂ O₃ alloys upon loss of some Y₂O₃, inasmuch as the production of the monoclinic phase of ZrO₂ is muchreduced.

Finally, I have found that, where desired, vanadium can be replaced withniobium and/or tantalum. Nevertheless, where resistance to chemicalcorrosion by vanadium/vanadium compounds at elevated temperatures issought, and the further factor that vanadium is less expensive thaneither niobium or tantalum, has limited such substitutions to less thanone-half of the vanadium content.

The third object can be attained in:

A ceramic alloy consisting essentially, expressed in terms of molepercent on the oxide basis, of:

(A) 40-94.75% total of at least one member selected from the groupconsisting of ZrO₂, HfO₂, partially stabilized ZrO₂, partiallystabilized HfO₂, ZrO₂ -HfO₂ solid solution, and partially stabilizedZrO₂ -HfO₂ solid solution;

(B) 5-45% SnO₂ ; and

(C) 0.25-15% total of at least one oxide in the indicated proportionselected from the group consisting of 0-10% Sc₂ O₃, 0-10% Y₂ O₃, 0-15%CeO₂, and 0-10% RE₂ O₃, wherein RE₂ O₃ is a rare earth metal oxideselected from the group consisting of La₂ O₃, Ce₂ O₃, Pr₂ O₃, Nd₂ O₃,Sm₂ O₃, Eu₂ O₃, Gd₂ O₃, Tb₂ O₃, Dy₂ O₃, Ho₂ O₃, Er₂ O₃, Tm₂ O₃, Yb₂ O₃,and Lu₂ O₃.

As was explained above, SnO₂ can replace TiO₂ and vice versa; suchreplacements generally being limited to no more than one-half of thecontent. Likewise, it may be desirable to replace up to about one-halfof the SnO₂ here with TiO₂.

The microstructure of the alloys toughened/stabilized with SnO₂ willconsist of fine-grained crystals of mixed phases commonly havingdiameters of less than about five microns consisting essentially eitherof tetragonal and monoclinic structures, or having a crystal phaseconsisting essentially of tetragonal symmetry with a minor amount of acubic phase and/or a ZrSnO₄ phase.

The inventive ceramic alloys are useful in enhancing the toughness ofhard refractory ceramic bodies. That is, as little as 5% by volume ofthese alloys can impart substantially improved toughness to such bodies.The ceramic matrix, which can comprise up to 95% by volume, may includesuch materials as α-Al₂ O₃, β-Al₂ O₃, β"-Al₂ O₃, Al₂ O₃ -Cr₂ O₃ solidsolutions, mullite, nasicon, sialon, SiC, TiB₂, Si₃ N₄, spinel, TiC, Al₂O₃ -mullite/Cr₂ O₃ -mullite solid solutions, zircon, and ZrC.

At high concentrations of α-Al₂ O₃, β-Al₂ O₃, β"-Al₂ O₃, and Al₂ O₃ -Cr₂O₃ solid solutions, that is, concentrations greater than about 70% byvolume, a surprising and, as yet, not understood phenomenon occurs. Noadditional stabilizing agent as such appears to be necessary in thealloy. Thus, there appears to be no need for the 0.25-15% total ofstabilizer oxide in the tungstate- and molybdate-containing alloys orthe 0.25-15% total of stabilizer oxide in the SnO₂ -containing alloys.(It will be appreciated, of course, that the presence of stabilizers inthe alloys does not adversely affect the properties of the resultanthard refractory ceramic bodies containing high levels of Al₂ O₃ and/orAl₂ O₃ -Cr₂ O₃. Nevertheless, the cost of the stabilizers is typicallyquite high; hence, their absence will usually result in a reduction ofcost in the final product.)

The inventive ceramic alloys are also effective in producing toughcomposite bodies wherein refractory ceramic fibers and/or whiskers maycomprise up to 80% by volume of the final product. Examples of operablefibers and/or whiskers include Al₂ O₃, AlN, BN, B₄ C, mullite, SiC, Si₃N₄, silicon oxycarbide, sialon, spinel, zircon, and ZrO₂.

Finally, a tough, three-component, composite ceramic body can beproduced consisting essentially of the inventive alloy, a hardrefractory ceramic, and refractory ceramic fibers and/or whiskers. Insuch a body the alloy will compose at least 5% by volume, refractoryceramic fibers and/or whiskers are present in an amount not exceeding80% by volume, and a hard refractory ceramic will comprise theremainder, usually in an amount of at least 5% by volume.

Prior Art

As has been explained above, stabilizer additives have been frequentlyemployed in the ceramic art to restrain otherwise uncontrollable phasetransformations. Hence, various dopants have been utilized to arrest thetransformation reaction at a point in its development where theresulting phase or mixture of phases renders the material most useful.For example, moderate concentrations of MgO, CaO, Y₂ O₃, CeO₂, and otherrare earth oxides have been incorporated into ZrO₂ to yield stabilizedand partially stabilized bodies demonstrating very interestingproperties. ZrO₂ -Y₂ O₃ systems are particularly attractive because ofthe direct dependence of the tetragonal-to-monoclinic transformationtemperature on the amount of Y₂ O₃ present. Such systems, however, didnot produce articles exhibiting excellent toughening characteristics,and Y₂ O₃ is a very expensive ingredient.

U.S. Pat. No. 3,957,500 describes the preparation of stabilized ZrO₂ byadding an impure Y₂ O₃ concentrate containing Y₂ O₃, heavy and lightrare earth metal oxides, and some incidental impurities. Whereas the useof rare earth metal oxides is common to both that invention and thepresent invention, no reference is made to the utility of MgWO₄ and/orMgMoO₄ and/or CaWO₄ and/or CaMoO₄, WO₃ and/or MoO₃, MVO₄, and SnO₂ astoughening/stabilizing agents.

U.S. Pat. No. 4,226,979 discloses an oxygen sensor cement prepared bycombining ZrO₂ powder with 4-8 mole % Y₂ O₃ powder and then molding andfiring the shaped mixture at 1400°-1500° C. Again, there is no referenceto the utility of MgWO₄ and/or MgMoO₄ and/or CaWO₄ and/or CaMoO₄, WO₃and/or MoO₃, MVO₄, and SnO₂ as toughening/stabilizing agents.

The use of a combination of stabilizers with ZrO₂, rather than a singlecompound, is also known to the art. To illustrate:

U.S. Pat. No. 4,303,447 outlines means for promoting the densificationof ZrO₂ stabilized with CaO or Y₂ O₃ at temperatures below 1300° C.,desirably below 1150° C., through the addition of B₂ O₃ or V₂ O₅thereto. There is no discussion of the capability of improving thetoughness of ZrO₂ bodies by incorporating MgWO₄ and/or MgMoO₄ and/orCaWO₄ and/or CaMoO₄, WO₃ and/or MoO₃, MVO₄, or SnO₂ therein.

U.S. Pat. No. 4,598,053 relates to a ZrO₂ body with primary stabilizersselected from the group of oxides consisting of CaO, MgO, La₂ O₃, Sc₂O₃, Y₂ O₃, and mixtures thereof, and a secondary component consisting ofat least one member of the group consisting of carbonitrides,carboxynitrides, oxynitrides, and oxycarbides of Groups IVa, Va, and VIaof the Periodic Table. Again, there is no mention of MgWO₄ and/or MgMoO₄and/or CaWO₄ and/or CaMoO₄, WO₃ and/or MoO₃, MVO₄, and SnO₂ astoughening/stabilizing agents.

U.S. Pat. No. 4,659,680 is concerned with a ZrO₂ body stabilized with0.5-10% Y₂ O₃ and 1-10% of a secondary stabilizer selected from thegroup consisting of CaO, CeO₂, CuO, MgO, and ZnO. Yet again, there is noreference to MgWO₄ and/or MgMoO₄ and/or CaWO₄ and/or CaMoO₄, WO₃ and/orMoO₃, MVO₄, and/or SnO₂ as toughening/stabilizing agents.

European Pat. No. 140,638 presents a ZrO₂ -containing sintered bodyconsisting of 50-98 weight % ZrO₂ stabilized with 1.5-5 mole % Y₂ O₃,and 2-50 weight % Al₂ O₃, mullite, or spinel. Once again, there is noindication of the utility of MgWO₄ and/or MgMoO₄ and/or CaWO₄ and/orCaMoO₄, WO₃ and/or MoO₃, MVO₄, and SnO₂ as toughening/stabilizingagents.

Two recent disclosures relating to the transformation toughening of ZrO₂bodies have been discussed in some detail above. U.S. Pat. No. 4,753,902and Ser. No. 926,655 are incorporated by reference into the presentdisclosure. There is no mention in either disclosure, however, of theuse of MgWO₄ and/or MgMoO₄ and/or CaWO₄ and/or CaMoO₄, WO₃ and/or MoO₃,MVO₄, and SnO₂ as toughening/stabilizing agents.

Description of Preferred Embodiments

The following procedure was used in synthesizing thepartially-stabilized, transformation toughened ceramic alloys describedin Table I:

Approximately 20 g of commercial zirconia (ZrO₂ with 2, 3, 4, and 6 mole% Y₂ O₃), and appropriate amounts of calcium carbonate and tungstenoxide and/or molybdenum oxide were mixed together by ballmilling in 250ml nalgene bottles utilizing approximately 45 zirconia balls of 0.5 inchdiameter as the milling media. Isopropyl alcohol was added to cover thepowder and milling media and the bottles placed into vibromillingcanisters and milled for approximately 24 hours. The resulting slurrywas poured into Pyrex drying dishes and air dried in a drying oven at320° F.

After drying, the powders were poured into alumina crucibles, partiallycovered, and calcined in air at 800° C. for 2 hours. In the case ofExample 21, the powder was then combined with nitrate salts of eithergadolinium and ytterbium, and the mixture was mixed into enough methanolto make a slurry. The slurry was then dried in a drying oven, calcinedat 800° C. for 2 hours, and vibromilled for 24 hours. After milling, thepowders were scalped through a nylon screen to break up anyagglomerates, reducing the agglomerate size to 50μ or less.

The resulting fine grain powder was pressed into pills, first uniaxiallyat 1000 psi in a 0.5 inch diameter die and then isostatically at 45 Kpsifor 10 minutes. One pill of each composition was fired at 1300° C.,1400° C., and 1500° C. for 2 hours and examined for completeness ofsintering.

The sintered specimens were then ground, polished, and microhardnesstested utilizing a 10 kg load. The Young's modulus, denoted by theletter E, was assumed to be 200 GPa unless the measured hardness waslower than 7 GPa. If so, the elastic modulus was multiplied by themeasured hardness and divided by 11 GPa, assuming that the modulusdecreases in proportion to the hardness. The elastic modulus willdecrease with porosity and microcracking which is reflected in a largedecrease in hardness.

The toughness, K_(IC), and hardness, H, were calculated from thefollowing equations:

    K.sub.IC =0.016(E.sup.0.5 P.sup.0.5 dC.sup.-1.5)

where E is 200 GPa, P is the 10 kg load, d is the length of the indentdiagonal, and C is the crack length from the center of the indentimpression, and

    H=1.845P/d.sup.2.

These equations were utilized in formulating the hardness and toughnessdata in the following tables.

                                      TABLE I                                     __________________________________________________________________________     Example                                                                              (ZrO.sub.2 +)Composition                                                                    Temp. (°C.)Sintering                                                           GpaHardness                                                                       ##STR1##                                    __________________________________________________________________________    1     3 m % Y.sub.2 O.sub.3                                                                        1300   13.7 4.8                                          1     3 m % Y.sub.2 O.sub.3                                                                        1400   13.1 5.1                                          1     3 m % Y.sub.2 O.sub.3                                                                        1500   13.7 5.7                                          2     3 m % Y.sub.2 O.sub.3 +                                                                      1300    9.9 6.1                                                0.5 m % CaWO.sub.4                                                      2     3 m % Y.sub.2 O.sub.3 +                                                                      1400   13.1 5.0                                                0.5 m % CaWO.sub.4                                                      2     3 m % Y.sub.2 O.sub.3 +                                                                      1500   12.6 7.4                                                0.5 m % CaWO.sub.4                                                      3     3 m % Y.sub.2 O.sub.3 +                                                                      1300    9.9 6.3                                                2.5 m % CaWO.sub.4                                                      3     3 m % Y.sub.2 O.sub.3 +                                                                      1400   12.0 6.7                                                2.5 m % CaWO.sub.4                                                      3     3 m % Y.sub.2 O.sub. 3 +                                                                     1500   11.6 6.0                                                2.5 m % CaWO.sub.4                                                      4     3 m % Y.sub.2 O.sub.3 +                                                                      1300   10.7 6.3                                                5.0 m % CaWO.sub.4                                                      4     3 m % Y.sub.2 O.sub.3 +                                                                      1400   11.6 6.5                                                5.0 m % CaWO.sub.4                                                      4     3 m % Y.sub.2 O.sub.3 +                                                                      1500    9.9 5.6                                                5.0 m % CaWO.sub.4                                                      5     3 m % Y.sub.2 O.sub.3 +                                                                      1300    8.0 5.9                                                10 m % CaWO.sub.4                                                       5     3 m % Y.sub.2 O.sub.3 +                                                                      1400   11.1 5.6                                                10 m % CaWO.sub.4                                                       5     3 m % Y.sub.2 O.sub.3 +                                                                      1500   10.7 5.6                                                10 m % CaWO.sub.4                                                       6     3 m % Y.sub.2 O.sub.3 +                                                                      1300   Porous                                                                             Porous                                             0.5 m % CaMoO.sub.4                                                     6     3 m % Y.sub.2 O.sub.3 +                                                                      1400    8.5 5.4                                                0.5 m % CaMoO.sub.4                                                     6     3 m % Y.sub.2 O.sub.3 +                                                                      1500   12.0 7.3                                                0.5 m % CaMoO.sub.4                                                     7     3 m % Y.sub.2 O.sub.3 +                                                                      1300    6.0 5.2                                                2.5 m % CaMoO.sub.4                                                     7     3 m % Y.sub.2 O.sub.3 +                                                                      1400   11.6 5.6                                                2.5 m % CaMoO.sub.4                                                     7     3 m % Y.sub.2 O.sub.3 +                                                                      1500   11.6 5.5                                                2.5 m % CaMoO.sub.4                                                     8     3 m % Y.sub.2 O.sub.3 +                                                                      1300    5.7 5.2                                                5.0 m % CaMoO.sub.4                                                     8     3 m % Y.sub.2 O.sub.3 +                                                                      1400   11.6 6.2                                                5.0 m % CaMoO.sub.4                                                     8     3 m % Y.sub.2 O.sub.3 +                                                                      1500    8.6 5.5                                                5.0 m % CaMoO.sub.4                                                     9     3 m % Y.sub.2 O.sub.3 +                                                                      1300   Porous                                                                             Porous                                             10 m % CaMoO.sub.4                                                      9     3 m % Y.sub.2 O.sub.3 +                                                                      1400    9.6 6.0                                                10 m % CaMoO.sub.4                                                      9     3 m % Y.sub.2 O.sub.3 +                                                                      1500   microcracked                                            10 m % CaMoO.sub.4                                                      10    3 m % Y.sub.2 O.sub.3 +                                                                      1300   porous                                                                             porous                                             1.25 m % CaWO.sub.4 +                                                         1.25 m % CaMoO.sub.4                                                    10    3 m % Y.sub.2 O.sub.3 +                                                                      1400   11.6 6.6                                                1.25 m % CaWO.sub.4 +                                                         1.25 m % CaMoO.sub.4                                                    10    3 m % Y.sub.2 O.sub.3 +                                                                      1500   12.6 8.2                                                1.25 m % CaWO.sub.4 +                                                         1.25 m % CaMoO.sub.4                                                    11    2 m % Y.sub.2 O.sub.3                                                                        1400   12.6 13.4                                         11    2 m % Y.sub.2 O.sub.3                                                                        1500   11.6 15.1                                         12    2 m % Y.sub.2 O.sub.3 +                                                                      1400   11.6 14.3                                               2.5 m % CaWO.sub.4                                                      12    2 m % Y.sub.2 O.sub.3 +                                                                      1500   11.6 15.9                                               2.5 m % CaWO.sub.4                                                      13    2 m % Y.sub.2 O.sub.3 +                                                                      1400   11.6 9.4                                                5.0 m % CaWO.sub.4                                                      13    2 m % Y.sub.2 O.sub.3 +                                                                      1500   11.6 13.6                                               5.0 m % CaWO.sub.4                                                      14    2 m % Y.sub.2 O.sub.3 +                                                                      1400   10.7 6.1                                                10 m % CaWO.sub.4                                                       14    2 m % Y.sub.2 O.sub.3 +                                                                      1500    9.2 14.5                                               10 m % CaWO.sub.4                                                       15    4 m % Y.sub.2 O.sub.3                                                                        1300   13.7 3.6                                          15    4 m % Y.sub.2 O.sub.3                                                                        1400   13.7 3.5                                          15    4 m % Y.sub.2 O.sub.3                                                                        1500   12.6 4.5                                          16    4 m % Y.sub.2 O.sub.3 +                                                                      1300    9.9 4.6                                                2.5 m % CaWO.sub.4                                                      16    4 m % Y.sub.2 O.sub.3 +                                                                      1400   13.7 4.2                                                2.5 m % CaWO.sub.4                                                      16    4 m % Y.sub.2 O.sub.3 +                                                                      1500   12.6 5.1                                                2.5 m % CaWO.sub.4                                                      17    4 m % Y.sub.2 O.sub.3 +                                                                      1300   11.6 4.0                                                3.0 m % WO.sub.3                                                        17    4 m % Y.sub.2 O.sub.3 +                                                                      1400   11.6 4.9                                                3.0 m % WO.sub.3                                                        17    4 m % Y.sub.2 O.sub.3 +                                                                      1500    9.9 6.5                                                3.0 m % WO.sub.3                                                        18    4 m % Y.sub.2 O.sub.3 +                                                                      1300   porous                                                                             porous                                             3.0 m % MoO.sub.3                                                       18    4 m % Y.sub.2 O.sub.3 +                                                                      1400    6.6 4.5                                                3.0 m % MoO.sub.3                                                       18    4 m % Y.sub.2 O.sub.3 +                                                                      1500   11.6 7.0                                                3.0 m % MoO.sub.3                                                       19    6 m % Y.sub.2 O.sub.3                                                                        1300   13.7 2.5                                          19    6 m % Y.sub.2 O.sub.3                                                                        1400   13.7 2.6                                          19    6 m % Y.sub.2 O.sub.3                                                                        1500   13.1 2.9                                          20    6 m % Y.sub.2 O.sub.3 +                                                                      1300    8.0 2.5                                                2.5 m % CaWO.sub.4                                                      20    6 m % Y.sub.2 O.sub.3 +                                                                      1400   12.6 2.7                                                2.5 m % CaWO.sub.4                                                      20    6 m % Y.sub.2 O.sub.3 +                                                                      1500   11.6 3.3                                                2.5 m % CaWO.sub.4                                                      21    3.8 m % (Yb.sub.2 O.sub.3,Gd.sub.2 O.sub.3) +                                                1300   11.6 3.7                                                2.5 m % WO.sub.3                                                        21    3.8 m % (Yb.sub.2 O.sub.3,Gd.sub.2 O.sub.3) +                                                1400   11.6 3.8                                                2.5 m % WO.sub.3                                                        21    3.8 m % (Yb.sub.2 O.sub.3,Gd.sub.2 O.sub.3) +                                                1500    9.9 10.1                                               2.5 m % WO.sub.3                                                        __________________________________________________________________________

The hardness and toughness data shown in Table I indicate that goodvalues of hardness and toughness can be obtained for compositionscontaining as little as 0.5 mole percent calcium tungstate/molybdate. Inyttria-stabilized zirconia, the toughness increased with decreasingyttria content while the hardness decreased gradually with increasingyttria content. Yttria-stabilized zirconia was used as a reference pointto which toughness and hardness values were compared as the amount oftoughening agent was increased.

At constant yttria mole percent, additions of 0.5-10 mole percentcalcium tungstate increased toughness significantly. When stabilizedwith 3 mole percent yttria and toughened with 0.5-10 mole percentcalcium tungstate, toughness values of zirconia ranged from 5.9 to 6.3MPa√m at 1300° C., from 5.0 to 6.7 MPa√m at 1400° C., and from 5.6 to7.4 MPa√m at 1500° C. Comparable values of toughness for zirconiastabilized solely with 3 mole percent yttria were 4.8, 5.1, and 5.7 atMPa√m 1300° C., 1400° C., and 1500° C., respectively.

When stabilized with 2 mole percent yttria and toughened with 2.5-10mole percent calcium tungstate, toughness values of zirconia ranged from6.1 to 14.3 MPa√m 1400 ° C. and from 13.6 to 15.9 MPa√m at 1500° C.Comparable values of toughness for zirconia stabilized solely with 2mole percent yttria were 13.4 and 15.1 MPA√m at 1400° C. and 1500° C.,respectively.

When stabilized with 4 and 6 mole percent yttria and toughened withcalcium tungstate, toughness values of zirconia, again, exhibited asignificant increase over yttria-stabilized zirconia. The highesttoughness values were found when zirconia was stabilized with 2 molepercent yttria and toughened with 2.5 mole percent calcium tungstate.

Calcium molybdate exhibited similar toughening trends when 0.5-10 molepercent was added to zirconia stabilized with 3 mole percent yttria.Toughness values were about 5.2 MPa√m at 1300° C., and ranged from5.4-6.2 MPa√m at 1400° C., and from 5.5-7.3 MPa√m at 1500° C. Comparablevalues of toughness for zirconia stabilized solely with 3 mole percentyttria were 4.8, 5.1, and 5.7 MPa√m at 1300° C., 1400° C., and 1500° C.,respectively. The highest toughness values were found when zirconia wasstabilized with 3 mole percent yttria and toughened with 0.5 and 5.0mole percent calcium molybdate.

When doped with tungsten oxide, yttria-stabilized zirconia showedimprovements in toughness of 11%, 20%, and 44% at 1300° C., 1400° C.,and 1500° C., respectively, while additions of molybdenum oxide improvedtoughness 29% and 56% at 1400° C. and 1500° C., respectively. Thehardness showed a slight decrease when tungsten oxide or molybdenumoxide was added.

When 1.25 mole percent calcium tungstate and 1.25 mole percent calciummolybdate were added to yttria-stabilized zirconia, it exhibited 30% and40% increases in toughness at 1400° C. and 1500° C. respectively.Previous data on calcium tungstate and calcium molybdate hint thatcalcium tungstate rich combinations may yield higher toughness values.

Whereas the above description has been drawn to shaped bodies producedby such means as sintering, it will be recognized that the term bodyincludes such variants as beads, coatings, fibers, honeycombs, andsheets fabricated utilizing a wide variety of methods known to the art,including but not limited to, hot pressing, arc melting, plasmaspraying, skull melting, and zone melting. For example, the hardness andtoughness exhibited by the inventive materials strongly suggest theirutility as abrasion resistant and thermal barrier coatings.

The advantage of the disclosed invention lies in the markedly improvedtoughness of ceramic matrix materials stabilized with SnO₂, CeO₂, Sc₂O₃, Y₂ O₃, TiO₂, and/or RE₂ O₃ and toughened with vanadates, calciumtungstate and/or calcium molybdate and/or magnesium tungstate and/ormagnesium molybdate, and tungsten oxide and/or molybdenum oxide. Calciumtungstate/molybdate and magnesium tungstate/molybdate toughening agentsmake possible the production of ceramic alloys with good toughness, yetwithout great expense.

I claim:
 1. A transformation toughened ceramic alloy consistingessentially, expressed in terms of mole percent on the oxide basis,of(A) 79-99.5% of at least one member selected from the group consistingof ZrO₂, HfO₂, partially stabilized ZrO₂, partially stabilized HfO₂,ZrO₂ -HfO₂ solid solution, and partially stabilized ZrO₂ -HfO₂ solidsolution; (B) 0.25-15% of at least one stabilizer oxide in the indicatedproportions selected from the group consisting of 0-7% Sc₂ O₃, 0-7% Y₂O₃, 0-15% CeO₂, 0-15% TiO₂, and 0-7% RE₂ O₃, wherein RE₂ O₃ is a rareearth metal oxide selected from the group consisting of La₂ O₃, Ce₂ O₃,Pr₂ O₃, Nd₂ O₃, Sm₂ O₃, Eu₂ O₃, Gd₂ O₃, Tb₂ O₃, Dy₂ O₃, Ho₂ O₃, Er₂ O₃,Tm₂ O₃, Yb₂ O₃, and Lu₂ O₃ ; and (C) 0.25-6% of at least one tougheningagent in the indicated proportion selected from the group consisting of0-6% MgWO₄, 0-6% MgMoO₄, 0-6% CaWO₄, 0-6% CaMoO₄, 0-4% WO₃, and 0-4%MoO₃.
 2. A ceramic alloy according to claim 1 wherein up to one-half ofthe TiO₂ is replaced with SnO₂.
 3. A ceramic alloy exhibiting resistanceto attack by vanadium compounds at elevated temperatures consistingessentially, expressed in terms of mole percent on the oxide basis,of(A) 65-99.5% of at least one member selected from the group consistingof HfO₂, partially stabilized ZrO₂, partially stabilized HfO₂, ZrO₂-HfO₂ solid solution, and partially stabilized ZrO₂ -HfO₂ solidsolution; and (B) 0.5-35% MVO₄ wherein M consists of at least one cationselected from the group consisting of Mg⁺², Ca⁺², Sc⁺³, Y⁺³, Sn⁺⁴, Ti⁺⁴,La⁺³, Ce⁺³, Ce⁺⁴, Pr⁺³, Nd⁺³, Sm⁺³, Eu⁺³, Gd⁺³, Tb⁺³, Dy⁺³, Ho⁺³, Er⁺³,Tm⁺³, Yb⁺³, and Lu⁺³.
 4. A ceramic alloy according to claim 3 alsoexhibiting significant transformation toughening wherein said MVO₄ ispresent in an amount between 0.5-10%.
 5. A ceramic alloy according toclaim 4 wherein said MVO₄ is present in an amount between 0.5-5%.
 6. Aceramic alloy according to claim 3 wherein up to one-half of thevanadium content is replaced with niobium and/or tantalum.
 7. Atransformation toughened ceramic alloy consisting essentially, expressedin terms of mole percent on the oxide basis, of(A) 40-94.75% of at leastone member selected from the group consisting of ZrO₂, HfO₂, partiallystabilized ZrO₂, partially stabilized HfO₂, ZrO₂ -HfO₂ solid solution,and partially stabilized ZrO₂ -HfO₂ solid solution; (B) 5-45% SnO₂ ; and(C) 0.25-15% of at least one oxide in the indicated proportion selectedfrom the group consisting of 0-10% Sc₂ O₃, 0-10% Y₂ O₃, 0-15% CeO₂, and0-10% RE₂ O₃, wherein RE₂ O₃ is a rare earth metal oxide selected fromthe group consisting of La₂ O₃, Ce₂ O₃, Pr₂ O₃, Nd₂ O₃, Sm₂ O₃, Eu₂ O₃,Gd₂ O₃, Tb₂ O₃, Dy₂ O₃, Ho₂ O₃, Er₂ O₃, Tm₂ O₃, Yb₂ O₃, and Lu₂ O₃.
 8. Aceramic alloy according to claim 8 wherein up to one-half of the SnO₂ isreplaced with TiO₂.
 9. A ceramic body consisting essentially of 5-95% byvolume of a hard refractory ceramic material and 5-95% by volume of aceramic alloy consisting essentially, expressed in terms of mole percenton the oxide basis, of(A) 79-99.5% of at least one member selected fromthe group consisting of ZrO₂, HfO₂, partially stabilized ZrO₂, partiallystabilized HfO₂, ZrO₂ -HfO₂ solid solution, and partially stabilizedZrO₂ -HfO₂ solid solution; (B) 0.25-15% of at least one stabilizer oxidein the indicated proportions selected from the group consisting of 0-7%Sc₂ O₃, 0-7% Y₂ O₃, 0-15% CeO₂, 0-15% TiO₂, and 0-7% RE₂ O₃, wherein RE₂O₃ is a rare earth metal oxide selected from the group consisting of La₂O₃, Ce₂ O₃, Pr₂ O₃, Nd₂ O₃, Sm₂ O₃, Eu₂ O₃, Gd₂ O₃, Tb₂ O₃, Dy₂ O₃, Ho₂O₃, Er₂ O₃, Tm₂ O₃, Yb₂ O₃, and Lu₂ O₃ ; and (C) 0.25-6% of at least onetoughening agent in the indicated proportion selected from the groupconsisting of 0-6% MgWO₄, 0-6% MgMoO₄, 0-6% CaWO₄, 0-6% CaMoO₄, 0-4%WO₃, and 0-4% MoO₃.
 10. A ceramic body according to claim 9 wherein saidhard refractory ceramic material consists of at least one member of thegroup consisting of α-Al₂ O₃, β-Al₂ O₃, β"-Al₂ O₃, Al₂ O₃ -Cr₂ O₃ solidsolutions, mullite, nasicon, sialon, SiC, Si₃ N₄, spinel, TiB₂, TiC, Al₂O₃ -mullite/Cr₂ O₃ -mullite solid solutions, zircon, and ZrC.
 11. Aceramic body according to claim 10 wherein up to one-half of the TiO₂ insaid alloy is replaced with SnO₂.
 12. A ceramic body consistingessentially of greater than 70% and up to 95% by volume of hardrefractory ceramic material selected from the group consisting of α-Al₂O₃, β-Al₂ O₃, β"-Al₂ O₃, and Al₂ O₃ -Cr₂ O₃ solid solutions, and theremainder a ceramic alloy consisting essentially, expressed in terms ofmole percent on the oxide basis, of(A) 94-99.75% of at least one memberselected from the group consisting of ZrO₂, HfO₂, partially stabilizedZrO₂, partially stabilized HfO₂, ZrO₂ -HfO₂ solid solution, andpartially stabilized ZrO₂ -HfO₂ solid solution; and (B) 0.25-6% of atleast one toughening agent in the indicated proportion selected from thegroup consisting of 0-6% MgWO₄, 0-6% MgMoO₄, 0-6% CaWO₄, 0-6% CaMoO₄,0-4% WO₃, and 0-4% MoO₃.
 13. A ceramic body consisting essentially of upto 80% by volume refractory ceramic fibers and/or whiskers and theremainder of a ceramic alloy consisting essentially, expressed in termsof mole percent on the oxide basis, of(A) 79-99.5% of at least onemember selected from the group consisting of ZrO₂, HfO₂, partiallystabilized ZrO₂, partially stabilized HfO₂, ZrO₂ -HfO₂ solid solution,and partially stabilized ZrO₂ -HfO₂ solid solution; (B) 0.25-15% of atleast one stabilizer oxide in the indicated proportions selected fromthe group consisting of 0-7% Sc₂ O₃, 0-7% Y₂ O₃, 0-15% CeO₂, 0-15% TiO₂,and 0-7% RE₂ O₃, wherein RE₂ O₃ is a rare earth metal oxide selectedfrom the group consisting of La₂ O₃, Ce₂ O₃, Pr₂ O₃, Nd₂ O₃, Sm₂ O₃, Eu₂O₃, Gd₂ O₃, Tb₂ O₃, Dy₂ O₃, Ho₂ O₃, Er₂ O₃, Tm₂ O₃, Yb₂ O₃, and Lu₂ O₃ ;and (C) 0.25-6% of at least one toughening agent in the indicatedproportion selected from the group consisting of 0-6% MgWO₄, 0-6%MgMoO₄, 0-6% CaWO₄, 0-6% CaMoO₄, 0-4% WO₃, and 0-4% MoO₃.
 14. A ceramicbody according to claim 13 wherein said refractory ceramic fibers and/orwhiskers consist of at least one member of the group consisting of Al₂O₃, AlN, BN, B₄ C, mullite, SiC, Si₃ N₄, silicon oxycarbide, sialonspinel, zircon, and ZrO₂.
 15. A ceramic body according to claim 13wherein up to one-half of the TiO₂ in said alloy is replaced with SnO₂.16. A three-component, composite ceramic body consisting of a hardrefractory ceramic material, refractory ceramic fibers and/or whiskers,and a ceramic alloy consisting essentially, expressed in terms of molepercent on the oxide basis, of(A) 79-99.5% of at least one memberselected from the group consisting of ZrO₂, HfO₂, partially stabilizedZrO₂, partially stabilized HfO₂, ZrO₂ -HfO₂ solid solution, andpartially stabilized ZrO₂ -HfO₂ solid solution; (B) 0.25-15% of at leastone stabilizer oxide in the indicated proportions selected from thegroup consisting of 0-7% Sc₂ O₃, 0-7% Y₂ O₃, 0-15% CeO₂, 0-15% TiO₂, and0-7% RE₂ O₃, wherein RE₂ O₃ is a rare earth metal oxide selected fromthe group consisting of La₂ O₃, Ce₂ O₃, Pr₂ O₃, Nd₂ O₃, Sm₂ O₃, Eu₂ O₃,Gd₂ O₃, Tb₂ O₃, Dy₂ O₃, Ho₂ O₃, Er₂ O₃, Tm₂ O₃, Yb₂ O₃, and Lu₂ O₃ ; and(C) 0.25-6% of at least one toughening agent in the indicated proportionselected from the group consisting of 0-6% MgWO₄, 0-6% MgMoO₄, 0-6%CaWO₄, 0-6% CaMoO₄, 0-4% WO₃, and 0-4% MoO₃ ;said refractory ceramicfibers and/or whiskers being present in an amount not exceeding 80% byvolume, said ceramic alloy being present in an amount of at least 5% byvolume, and said refractory ceramic material comprising the remainder ofsaid body.
 17. A ceramic body according to claim 16 wherein said hardrefractory ceramic material consists of at least one member of the groupconsisting of α-Al₂ O₃, β-Al₂ O₃, β"-Al₂ O₃, Al₂ O₃ -Cr₂ O₃ solidsolutions, mullite, nasicon, sialon, SiC, Si₃ N₄, spinel, TiB₂, TiC, Al₂O₃ -mullite/Cr₂ O₃ -mullite solid solutions, zircon, and ZrC, and saidrefractory ceramic fibers and/or whiskers consist of at least one memberof the group consisting of Al₂ O₃, AlN, BN, B₄ C, mullite, SiC, Si₃ N₄,silicon oxycarbide, sialon, spinel, zircon, and ZrO₂.
 18. A ceramic bodyaccording to claim 16 wherein up to one-half of the TiO₂ in said alloyis replaced with SnO₂.
 19. A ceramic body consisting essentially of5-95% by volume of a hard refractory ceramic material and 5-95% byvolume of a ceramic alloy consisting essentially, expressed in terms ofmole percent on the oxide basis, of(A) 65-99.5% of at least one memberselected from the group consisting of HfO₂, partially stabilized ZrO₂,partially stabilized HfO₂, ZrO₂ -HfO₂ solid solution, and partiallystabilized ZrO₂ -HfO₂ solid solution; and (B) 0.5-35% MVO₄, wherein Mconsists of at least one cation selected from the group consisting ofMg⁺², Ca⁺², Sc⁺³, Y⁺³, Sn⁺⁴, Ti⁺⁴, La⁺³, Ce⁺³, Ce⁺⁴, Pr⁺³, Nd⁺³, Sm⁺³,Eu⁺³, Gd⁺³, Tb⁺³, Dy⁺³, Ho⁺³, Er⁺³, Tm⁺³, Yb⁺³, and Lu⁺³.
 20. A ceramicbody according to claim 19 wherein said MVO₄ of said alloy is present inan amount between 0.5-10%.
 21. A ceramic body according to claim 19wherein up to one-half of the vanadium content in said alloy is replacedwith niobium and/or tantalum.
 22. A ceramic body according to claim 19wherein said hard refractory ceramic material consists of at least onemember of the group consisting of α-Al₂ O₃, β-Al₂ O₃, β"-Al₂ O₃, Al₂ O₃-Cr₂ O₃ solid solutions, mullite, nasicon, sialon, SiC, Si₃ N₄, spinel,TiB₂, TiC, Al₂ O₃ -mullite/Cr₂ O₃ -mullite solid solutions, zircon, andZrC.
 23. A ceramic body consisting essentially of up to 80% by volume ofrefractory ceramic fibers and/or whiskers and the remainder of a ceramicalloy consisting essentially, expressed in terms of mole percent on theoxide basis, of(A) 65-99.5% of at least one member selected from thegroup consisting of ZrO₂, HfO₂, partially stabilized ZrO₂, partiallystabilized HfO₂, ZrO₂ -HfO₂ solid solution, and partially stabilizedZrO₂ -HfO₂ solid solution; and (B) 0.5-35% MVO₄, wherein M consists ofat least one cation selected from the group consisting of Mg⁺², Ca⁺²,Sc⁺³, Y⁺³, Sn⁺⁴, Ti⁺⁴, and a rare earth metal cation selected from thegroup consisting of La⁺³, Ce⁺³, Ce⁺⁴, Pr⁺³, Nd⁺³, Sm⁺³, Eu⁺³, Gd⁺³,Tb⁺³, Dy⁺³, Ho⁺³, Er⁺³, Tm⁺³, Yb⁺³, and Lu⁺³.
 24. A ceramic bodyaccording to claim 23 wherein said refractory fibers and/or whiskersconsist of at least one member of the group consisting of Al₂ O₃, AlN,BN, B₄ C, mullite, SiC, Si₃ N₄, silicon oxycarbide, sialon, spinel,zircon, and ZrO₂.
 25. A ceramic body according to claim 24 wherein up toone-half of the vanadium content in said alloy is replaced with niobiumand/or tantalum.
 26. A three-component, composite ceramic bodyconsisting of a hard refractory ceramic material, refractory ceramicfibers and/or whiskers, and a ceramic alloy consisting essentially,expressed in terms of mole percent on the oxide basis of(A) 65-99.5% ofat least one member selected from the group consisting of HfO₂,partially stabilized ZrO₂, partially stabilized HfO₂, ZrO₂ -HfO₂ solidsolution, and partially stabilized ZrO₂ -HfO₂ solid solution; and (B)0.5-35% MVO₄, wherein M consists of at least one cation selected fromthe group consisting of Mg⁺², Ca⁺², Sc⁺³, Y⁺³, Sn⁺⁴, Ti⁺⁴, La⁺³, Ce⁺³,Ce⁺⁴, Pr⁺³, Nd⁺³, Sm⁺³, Eu⁺³, Gd⁺³, Tb⁺³, Dy⁺³, Ho⁺³, Er⁺³, Tm⁺³, Yb⁺³,and Lu⁺³ ;said refractory ceramic fibers and/or whiskers being presentin an amount not exceeding 80% by volume, said ceramic alloy beingpresent in an amount of at least 5% by volume, and said refractoryceramic material comprising the remainder of said body.
 27. A ceramicbody according to claim 26 wherein said hard refractory ceramic materialconsists of at least one member of the group consisting of α-Al₂ O₃,β-Al₂ O₃, β"-Al₂ O₃, Al₂ O₃ -Cr₂ O₃ solid solutions, mullite, nasicon,sialon, SiC, Si₃ N₄, spinel, TiB₂, TiC, Al₂ O₃ -mullite/Cr₂ O₃ -mullitesolid solutions, zircon, and ZrC, and said refractory ceramic fibersand/or whiskers consist of at least one member of the group consistingof Al₂ O₃, AlN, BN, B₄ C, mullite, SiC, Si₃ N₄, silicon oxycarbide,sialon, spinel, zircon, and ZrO₂.
 28. A ceramic body according to claim26 wherein up to one-half of the vanadium content in said alloy isreplaced with niobium and/or tantalum.
 29. A ceramic body consistingessentially of 5-95% by volume of a hard refractory ceramic material and5-95% by volume of a ceramic alloy consisting essentially, expressed interms of mole percent on the oxide basis, of(A) 40-94.75% of at leastone member selected from the group consisting of ZrO₂, HfO₂, partiallystabilized ZrO₂, partially stabilized HfO₂, ZrO₂ -HfO₂ solid solution,and partially stabilized ZrO₂ -HfO₂ solid solution; (B) 5-45% SnO₂ ; and(C) 0.25-15% of at least one oxide in the indicated proportion selectedfrom the group consisting of 0-10% So₂ O₃, 0-10% Y₂ O₃, 0-15% CeO₂, and0-10% RE₂ O₃, wherein RE₂ O₃ is a rare earth metal oxide selected fromthe group consisting of La₂ O₃, Ce₂ O₃, Pr₂ O₃, Nd₂ O₃, Sm₂ O₃, Eu₂ O₃,Gd₂ O₃, Tb₂ O₃, Dy₂ O₃, Ho₂ O₃, Er₂ O₃, Tm₂ O₃, Yb₂ O₃, and Lu₂ O₃. 30.A ceramic body according to claim 29 wherein up to one-half of the SnO₂in said alloy is replaced with TiO₂.
 31. A ceramic body according toclaim 29 wherein said hard refractory ceramic material consists of atleast one member of the group consisting of α-Al₂ O₃, β-Al₂ O₃, β"-Al₂O₃. Al₂ O₃ -Cr₂ O₃ solid solutions, mullite, nasicon, sialon, SiC, Si₃N₄, spinel, TiB₂, TiC, Al₂ O₃ -mullite/Cr₂ O₃ -mullite solid solutions,zircon, and ZrC.
 32. A ceramic body consisting essentially of up to 80%by volume of refractory ceramic fibers and/or whiskers and the remainderof a ceramic alloy consisting essentially, expressed in terms of molepercent on the oxide basis, of(A) 40-94.75% of at least one memberselected from the group consisting of ZrO₂, HfO₂, partially stabilizedZrO₂, partially stabilized HfO₂, ZrO₂ -HfO₂ solid solution, andpartially stabilized ZrO₂ -HfO₂ solid solution; (B) 5-45% SnO₂ ; and (C)0.25-15% of at least one oxide in the indicated proportion selected fromthe group consisting of 0-10% Sc₂ O₃, 0-10% Y₂ O₃, 0-15% CeO₂, and 0-10%RE₂ O₃, wherein RE₂ O₃ is a rare earth metal oxide selected from thegroup consisting of La₂ O₃, Ce₂ O₃, Pr₂ O₃, Nd₂ O₃, Sm₂ O₃, Eu₂ O₃, Gd₂O₃, Tb₂ O₃, Dy₂ O₃, Ho₂ O₃, Er₂ O₃, Tm₂ O₃, Yb₂ O₃, and Lu₂ O₃.
 33. Aceramic body according to claim 32 wherein said refractory fibers and/orwhiskers consist of at least one member of the group consisting of Al₂O₃, AlN, BN, B₄ C, mullite, SiC, Si₃ N₄, silicon oxycarbide, sialon,spinel, zircon, and ZrO₂.
 34. A ceramic body according to claim 32wherein up to one-half of the SnO₂ in said alloy is replaced with TiO₂.35. A three-component, composite ceramic body consisting of a hardrefractory ceramic material, refractory ceramic fibers and/or whiskers,and a ceramic alloy consisting essentially, expressed in terms of molepercent on the oxide basis, of(A) 40-94.75% of at least one memberselected from the group consisting of ZrO₂, HfO₂, partially stabilizedZrO₂, partially stabilized HfO₂, ZrO₂ -HfO₂ solid solution, andpartially stabilized ZrO₂ -HfO₂ solid solution; (B) 5-45% SnO₂ ; and (C)0.25-15% of at least one oxide in the indicated proportion selected fromthe group consisting of 0-10% Sc₂ O₃, 0-10% Y₂ O₃, 0-15% CeO₂, and 0-10%RE₂ O₃, wherein RE₂ O₃ is a rare earth metal oxide selected from thegroup consisting of La₂ O₃, Ce₂ O₃, Pr₂ O₃, Nd₂ O₃, Sm₂ O₃, Eu₂ O₃, Gd₂O₃, Tb₂ O₃, Dy₂ O₃, Ho₂ O₃, Er₂ O₃, Tm₂ O₃, Yb₂ O₃, and Lu₂ O₃ ; saidrefractory ceramic fibers and/or whiskers being present in an amount notexceeding 80% by volume, said ceramic alloy being present in an amountof at least 5% by volume, and said refractory ceramic materialcomprising the remainder of said body.
 36. A ceramic body according toclaim 35 wherein said hard refractory ceramic material consists of atleast one member of the group consisting of α-Al₂ O₃, β-Al₂ O₃, β"-Al₂O₃, Al₂ O₃ -Cr₂ O₃ solid solutions, mullite, nasicon, sialon, SiC, Si₃N₄, spinel, TiB₂, TiC, Al₂ O₃ -mullite/Cr₂ O₃ -mullite solid solutions,zircon, and ZrC, and said refractory ceramic fibers and/or whiskersconsist of at least one member of the group consisting of Al₂ O₃, AlN,BN, B₄ C, mullite, SiC, Si₃ N₄, silicon oxycarbide, sialon, spinel,zircon, and ZrO₂.
 37. A ceramic body according to claim 35 wherein up toone-half of the SnO₂ in said alloy is replaced with TiO₂.